app: sent the mouse events to other window when there is a modal window

In one app thread, we can have multiple window trees. If one tree is
modaled, there is no reason to discard events to other windows.

This partly resolves the issue #39 on GitHub.
app: remove modal_object as it is useless

There can be multiple modal object in one app. So there is no need to
record the one. Let's embrace the fully async, event-driven world.
add test_cases/modal_win